Evaporation from snow covers in Eastern Canada

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Evaporation from snow covers in Eastern Canada

NATIONAL RESEARCH COUNCIL O F CANADA D I V I S I O N O F B U I L D I N G RESEARCH

EVAPORATION FROM SNOW COVERS I N EASTERN CANADA

by G. P. W i l l i a m s R e s e a r c h Paper No. 7 3 o f t h e D i v i s i o n o f B u i l d i n g R e s e a r c h O t t a w a

January

1959

PREFACE

!This paper was p r e s e n t e d b e f o r e t h e

E a s t e r n Snow Conference a t Vforcester, Massachusetts,

13

and

14

February 1958. The Proceedings of t h i s Conference have n o t been p u b l i s h e d and t h e D i v i s i o n

i s

i s s u i n g t h i s r e p o r t a s a r e s e a r c h a paper i n i t s s e r i e s . Ottawa J a n u a r y 1959 Robert P, Legget D i r e c t o r

EVAPORATION FROIJ SNOW COVERS I N EASTERN CANADA H y d r o l o g i s t s c h a r g e d w i t h t h e r e s p o n s i b i l i t y o f e s t i m a t i n g t h e amount o f r u n o f f f r o m m e l t i n g snow h a v e l o n g b e e n i n t e r e s t e d i n e s t i m a t i n g t h e amoun-t o f e v a p o r a t i o n . f r o m a snow c o v e r . A l t h o u g h t h e r e h a v e b e e n a number o f i n v e s - t i g a t i o n s o n snow e v a p o r a t i o n ,

it

h a s b e e n d i f f i c u l t t o f i n d a n a n a l y s i s t h a t g i v e s a n o r d e r - o f - m a g n i t u d e f o r e v a p o r a t i o n f r o m t h e c o m p a r a t i v e l y s h a l l o w snow c o v e r s t h a t a r e s u b j e c t t o t h e v i e a t h e r c o n d i t i o n s t y p i c a l o f E a s t e r n Canada. The p u r p o s e o f t h i s p a p e r i s t o r e v i e w t h e l i t e r a - t u r e a n d e x p e r i m e n t s on e v a p o r a t i o n f r o m snow c o v e r s i n o t h e r c o u n t r i e s a n d t o make a p r e l i m i n a r y e v a l u a t i o n o f some snow e v a p o r a - t i o n m e a s u r e m e n t s t a k e n a t O t t a w a d u r i n d t h e 1956-57 s e a s o n . The t h e o r y o f e v a p o r a t i o n f r o m snow c o v e r s w i l l n o t b e d i s c u s s e d i n t h i s r e p o r t ; Diamond h a s g i v e n a good a n a l y s i s o f t h e t h e o r y w h i c h would b e o f v a l u e t o e n g i n e e r s (1). S v e r d r u p ( 2 ) d e v e l o p e d a n e q u a t i o n f o r p r e d i c t i n g t h e r a t e o f e v a p o r a t i o n a n d m e l t f r o m a snow c o v e r w h i c h , f r o m a t h e o r e t i c a l v i e w p o i n t , i s s t i l l a c l a s s i c . L i g h t

( 3 )

u s e d S v e r d r u p v s e q u a t i o n f o r h e a t a n d w a t e r f l u x t o d e v e l o p an e q u a t i o n f o r p r e d i c t i n g t h e o r e t i c a l r a t e s o f snow evapo- r a t i o n .

Tlze b a s i c t h e o r y o f e v a p o r a t i o n from a snow c o v e r i s t h e same a s e v a p o r a t i o n f r o m a w a t e r s u r f a c e . Good r6sum6s o f t h i s t h e o r y , p a r t i c u l a r l y t h e t u r b u l e n t - e x c h a n g e t h e o r y o n w h i c h some e v a p o r a t i o n f o r m u l a s a r e b a s e d , a r e g i v e n by T h o r n t h w a i t e ( 4 ) a n d E.R, A n d e r s o n a n d o t h e r s ( 5 ) . The main p r o b l e m i n e v a p o r a t i o n s t u d i e s i s t o o b t a i n r e l i a b l e m e a s u r e m e n t s u n d e r f i e l d c o n d i t i o n s . E v a p o r a t i o n r a t e s f r o m o p e n w a t e r , b a r e s o i l o r g r a s s h a v e b e e n e s t i m a t e d by m e a s u r i n g t h e r a t e o f e v a p o r a t i o n from some s t a n d a r d p a n o r t a n k a n d f r o m t h e s e m e a s u r e m e n t s o b t a i n i n g a c o e f f i c i e n t w h i c h c o u l d b e u s e d t o e s t i m a t e e v a p o r a t i o n f r o m t h e s u r f a c e a r e a i n q u e s t i o n ( 6 ,

7 ) .

E v a p o r a t i 6 n r a t e s f r o m snow c o v e r s h a v e u s u a l l y b e e n m e a s u r e d b y f i l l i n g p a n s w i t h snovi a n d p l a c i n g them i n t o t h e snow c o v e r s o t h a t t h e s u r f a c e o f t h e p a n i s f l u s h w i t h t h e snow s u r f a c e . The p a n s a r e u s u a l l y p a i n t e d w h i t e o r a r e made o f p l a s t i c t o m i n i m i z e r a d i a t i o n e f f e c t s . The r a t e o f e v a p o r a t i o n f r o m t h e p a n i s assumed t o be r e p r e s e n t a t i v e o f t h a t f r o m t h e l a r g e r snow s u r f a c e a r e a .

d e Q u e r v a i n ( 8 ) f o u n d t h a t r a d i a t i o n d i d a f f e c t t h e e v a p o r a t i o n r a t e i f t h e p a n s w e r e t o o s h a l l o w . P a n s a l s o i n t e r f e r e w i t h t h e p o s s i b l e upviard f l o w o f v a p o u r t h r o u g h t h e snow c o v e r a n d t h e r e i s some q u e s t i o n a s t o what e f f e c t t h i s has o n t h e r a t e o f e v a p o r a t i o n from t h e snow s u r f a c e . It i s a l s o i m p o s s i b l e t o o b t a i n r e l i a b l e r e s u l t s from p a n s d u r i n g p e r i o d s o f p r e c i p i t a t i o n o r b l o w i n g snow. Some f i e l d measurements o f e v a p o r a t i o n w e r e made i n t h e P r a s e r E x p e r i - m e n t a l F o r e s t u s i n g a v o l u m e t r i c method a n d t h e pan method ( 9 1 , Because of d i f f i c u l t i e s o f measurement t h e y w e r e u n a b l e t o o b t a i n good c o m p a r i s o n s between t h e two m e t h o d s . It i s a p p a r e n t t h a t t h e u s e o f p a n s t o measure snow e v a p o r a t i o n has s e v e r a l drawbacks w h i c h make

it

d i f f i c u l t t o o b t a i n

c o n t i n u o u s , r e l i a b l e r e c o r d s . N e v e r t h e l e s s , t h e i r u s e a p p e a r s t o b e t h e o n l y way t h a t t h e o r d e r - o f - m a g n i t u d e o f snow evapo- r a t i o n r a t e s c a n b e a s c e r t a i n e d . L a t e r i n t h i s r e p o r t t h e p a n e v a p o r a t i o n mea.surements o f s e v e r a l w o r k e r s i n d i f f e r e n t c o u n t r i e s w i l l b e compared. The c o e f f i c i e n t s c a l c u l a t e d from t h e s e s t u d i e s f o r t h e v a r i o u s snow e v a p o r a t i o n f o r m u l a s w i l l b e compared w i t h t h e e v a p o r a t i o n c o e f f i c i e n t c a l c u l a t e d f r o m t h e snow e v a p o r a t i o n measurements t a k e n a t Ottawa d u r i n g t h e 1956-57 w i n t e r . F i e l d Measurements a t t h e O t t a w a S i t e Snow e v a p o r a t i o n w a s measured a t O t t a w a by p l a c i n g c i r c u l a r p a n s

( 4

cm d e e p w i t h 355 s q cm a r e a ) f i l l e d w i t h snow, f l u s h w i t h t h e snow s u r f a c e a n d w e i g h i n g t h e p a n s a t d i f f e r e n t t i m e i n t e r v a l s . The w e i g h t l o s s o r g a i n d u r i n g t h e t i m e p e r i o d w a s assumed t o be due t o e v a p o r a t i o n o r c o n d e n s a t i o n f r o m t h e snow s u r f a c e d u r i n g t h e p e r i o d o f o b s e r v a t i o n . The l i g h t aluminum p a n s w e r e p a i n t e d w h i t e t o r e d u c e t h e a b s o r p t i o n o f r a d i a t i o n f r o m t h e s u n . The o b s e r v a t i o n s i t e w a s i n an e x p o s e d l o c a t i o n n e a r a n a r e a where s t a n d a r d m e t e o r o l o g i c a l o b s e r v a t i o n s a r e made. The s t a n d a r d m e t e o r o l o g i c a l r e c o r d s o b t a i n e d a t t h i s s t a t i o n w e r e u s e d i n a l l c a l c u l a t i o n s . T h e r e w e r e many d a y s when

i t

w a s i m p o s s i b l e t o o b t a i n r e l i a b l e measurements b e c a u s e o f w e a t h e r c o n d i t i o n s . When- e v e r t h e r e w a s b l o w i n g snow, p r e c i p i t a t i o n , o r m e l t i n g snow, o b s e r v a t i o n s w e r e n o t t a k e n b e c a u s e o f u n c e r t a i n t y i n t h e measurements. F u r t h e r m o r e , d u r i n g t h e 1956-57 w i n t e r , t h e

d u r a t i o n o f t h e snow c o v e r w a s below n o r m a l i n t h e Ottawa a r e a a n d t h e o p p o r t u n i t i e s f o r o b s e r v a t i o n w e r e l i m i t e d .

A n a l y s i s o f R e s u l t s

F o r e a c h day d u r i n g w h i c h e v a p o r a t i o n w a s measured,

Q = 0 . 0 0 0 1 1 p Ua ( e

-

e s ) 2 Where Q = e v a p o r a t i o n r a t e , gm/cm /min

p = mean d e n s i t y o f a i r , gm/cm 5 Ua = mean viind s p e e d , cm/sec

( a t h e i g h t a cm) e = v a p o u r p r e s s u r e o f a i r i n S t e v e n s o n s c r e e n , ( m i l l i b a r s ) e = s a t u r a t e d v a p o u r p r e s s u r e o v e r i c e a t t h e mean t e m p e r a t u r e o f t h e snow s u r f a c e , ( m i l l i b a r s ) . Tile f o r m o f t h i s e q u a t i o n w a s d e v e l o p e d from t h e more c o m p l i c a t e d S v e r d r u p e q u a t i o n ( 2 ) by a s s u m i n g c o n s t a n t v a l u e s f o r s u r f a c e r o u g l ~ n e s s a n d a i r p r e s s u r e .

The mean win.d s p e e d w a s o b t a i n e d b y a v e r a g i n g t h e h o u r l y wind s p e e d v a l u e s d u r i n g t h e p e r i o d o f o b s e r v a t i o n , The v a p o u r p r e s s u r e o f t h e a i r was c a l c u l a t e d from t h e mean r e l a t i v e h u m i d i t y v a l u e r e c o r d e d a t 8 . 3 0 a . m . an.d a t 4 . 5 0 p.m. The v a p o u r p r e s s ~ ~ r e a t t h e snow s u r r a c e was c a l c u l a t e d b y a s s u m i n g t h a t t h e mean sriotv s u r l r a c e t e m p e r a t u r e i s e q u a l t o t h e mean a i r t e m p e r a t u r e r e c o r d e d i n t h e S t e v e n s o n s c r e e n . F i g u r e 1 shows t h e r e l a t i o n s h i p b e t w e e n t h e c a l c u l a t e d e v a p o r a t i o n r a t e a n d t h e m e a s u r e d e v a p o r a t i o n r a t e s a t t h e O t t a w a s i t e . The m e a s u r e d e v a p o r a t i o n r a t e i s t h e a v e r a g e r a t e f r o m t h e t h r e e e v a p o r a t i o n p a n s . T a b l e I l i s t s t h e r e s u l t s u s e d t o c a l c u l a t e t h e v a l u e s shown i n F i g . 1. C a l c u l a t e d E v z p o r a t i o r i R a t e s d u r i n g March The e v a p o r a t i o n o b s e r v a t i o n s l i s t e d i n T a b l e I w e r e a l l made d u r i n g t h e c o l d e r w i n t e r m o n t h s . D u r i n g t h e s p r i n g m e l t p e r i o d , w h i c h o c c u r r e d . i n March i n 1 9 5 7 , n o e v a p o r a t i o n measul-ements were made. Valenever m e l t i n g con- d i t i o n s o c c u r , w a t e r i s p r e s e n t w i t h t h e snow i n t h e pan a n d it i s d o u b t f u l w h e t h e r e v a p o r a t i o n r a t e s from t h i s m i x t u r e a r e t h e same a s e v a p o r a t i o n r a t e s f r o m a w e t snow

s u r f a c e where t h e f r e e v i a t e r c a n d r a i n downward. The o r d e r - o f - m a g n i t u d e of e v a p o r a t i o n r a t e s d u r i n g t h e s p r i n g

m e l t p e r i o d w a s e s t i m a t e d u s i n g e q u a t i o n 1 a n d t h e a p p r o p r i a t e m e t e o r o l o g i c a l d a t a . Wktenever t h e snow was m e l t i n g t h e

v a p o u r p r e s s u r e was assumed t o b e e q u a l t o t h e s a t u r a t e d v a p o u r p r e s s u r e o v e r i c e a t 32OF. C o n d e n s a t i o n w a s t r e a t e d as n e g a t i v e e v a p o r a t i o n .

The a v e r a g e c a l c u l a t e d e v a p o r a t i o n l o s s f r o m t h e snow c o v e r d u r i n g t h e month o f March f o r t h e y e a r s 1953-57 i n c l u - s i v e a r e p l o t t e d o n F i g . 2 . T h i s t o t a l m o n t h l y e v a p o r a t i o n l o s s w a s o b t a i n e d by a c c u m u l a t i n g t h e c a l c u l a t e d r a t e s f o r a 1 0 - h o u r d a y from 8 a . m . t o 6 p.m. B e c a u s e o f t h e u n c e r - t a i n t y of r e l a t i v e h u m i d i t y v a l u e s d u r i n g t h e n i g h t , t h e e v a p o r a t i o n r a t e s w e r e n o t c a l c u l a t e d f o r t h i s p e r i o d . I t i s g e n e r a l l y assumed t h a t e v a p o r a t i o n l o s s e s w i l l b e l e s s d u r i n g t h e n i g h t when t h e r e i s no h e a t s u p p l i e d by r a d i a t i o n a n d when t h e r e l a t i v e h u m i d i t y v a l u e s a r e h i g h e r t h a n d u r i n g t h e d a y l i g h t h o u r s . C r o f t ( 1 0 ) f o u n d t h a t e v a p o r a t i o n

l o s s e s from a snow c o v e r d u r i n g t h e d a y l i g h t h o u r s were t h r e e t o f o u r t i m e s a s g r e a t a s l o s s e s d u r i n g t h e n i e a t p e r i o d .

Thus F i g . 2 o n l y i n d i c a t e s t o t a l e v a p o r a t i o n l o s s e s d u r i n g p a r t o f t h e d a y a n d i s o n l y i n t e n d e d t o i n d i c a t e t h a t a v e r a g e l o s s e s f r o m a snow c o v e r a r e g e n e r a l l y u n d e r one i n c h o f w a t e r d u r i n g a month o f s p r i n g r u n o f f , a n d a r e

g e n e r a l l y much l e s s than t h e p r e c i p i t a t i o n d u r i n g t h a t month.

C o m ~ a r i s o n o f E v a ~ o r a t i o n C o e f f i c i e n t s The g e n e r a l f o r m o f e q u a t i o n . l i s t h e same a s t h a t u s e d b j ~ o t h e r i n v e s t i g a t o r s o f snow e v a p o r a t i o n . R e s e a r c h worlcers a t t h e C e n t r a l S i e r r a Snow L a b o r a t o r y (11) h a v e u s e d t h e f o l l o w i n g f o r m : Qe e q u a l s Ice ( e a

-

e s ) V b ; where Qe i s t h e amount o f e v a p o r a t i o n o r c o n d e n s a t i o n , ea a n d e a r e 9 t h e v a p o u r p r e s s u r e o f t h e a i r a n d t h e s a t u r a t e d v a p o u r p r e s s u r e o v e r i c e a t t h e mean snow s u r f a c e t e m p e r a t u r e r e s p e c t i v e l y , Vb i s t h e w i n d s p e e d , a n d Ice i s a c o n s t a n t . 1/6 I n a d d i t i o n , t h e y c a l c u l a t e d a Icef e q u a l t o 0 . 5 Ke ( Z a Z b )

,

where t h e s u b s c r i p t s a a n d b i d e n t i f y t h e l e v e l s o f m e a s u r e - ment o f t h e wind s p e e d a n d a i r v a p o u r p r e s s u r e r e s p e c t i v e l y , a n d K e ' i s t h e v a l u e o f t h e c o e f f i c i e n t when t h e wind s p e e d a n d v a p o u r p r e s s u r e a r e m e a s u r e d one f o o t a b o v e t h e snow s u r f a c e . They h a v e compared t h e e v a p o r a t i o n o r c o n d e n s a t i o n c o n s t a n t K e v f o r s e v e r a l w o r k e r s a s f o l l o w s : C e n t r a l S i e r r a Snow L a b o r a t o r y Snow I i i v e s t i g a t i o n s ( 1 9 5 1 ) 0.0540 S v e r d r u p ( 1 9 3 6 ) 0.0671 de Q u e r v a i n ( 1 9 5 1 ) 0.0770 By u s i n g t h e s a n e u n i t s o f v a p o u r p r e s s u r e i n m i l l i b a r s , mean w i n d s p e e d i n . m i l e s p e r h o u r , e v a p o r a t i o n o r c o n d e n s a t i o n i n i n c h e s , a n d h e i g h t s o f measurement i n f e e t , it was p o s s i b l e t o c a l c u l a t e a Icee' from t h e O t t - a w a

m e a s u r e m e n t s give11 i n T a b l e I . The mean X e f t h u s o b t a i n e d was O.C)1G w h i c h compares r e a s o n a b l y me11 w l t h t h e 0 . 0 5 1 v a l u e o b t a i n e d by t h e i n v e s t i g a t o r s a t t h e C e n t r a l S i e r r a Snow L a b o r a t o r y . The f a c t t h a t t h e s e c o e f f i c i e n t s a r e i n t h e same g e n e r a l r a n g e s t r e n g t h e n s t h e c o n f i d e n c e i n t h e u s e o f t h i s f o r m o f e q u a t i o n t o d e t e r m i n e t h e o r d e r - o f - m a g n i t u d e o f snow e v a p o r a t i o n r a t e s . c o m p a r i s o n o f Measured E v a p o r a t i o n - R a t e s T h e r e h a v e been. a s u r p r i s i n g number o f m e a s u r e m e n t s o f snow e v a p o r a t i o n r a t e s b u t , b e c a u s e o f t h e d i f f i c u l t i e s o f m e a s u r e m e n t , m o s t o f t h e r e c o r d s a r e s p o r a d i c a n d a l l t h e r e s u l t s

must

b e q u a l i f i e d b e c a u s e o f t l i e s e d i f f i c u l t i e s . A l t h o u g h i t i s d i f f i c u l t t o compare e v a p o l - a t i o n r a t e s b e c a u s e o f d i f f e r e n c e s i n s i t e s a n d wea-thor c o n d i t i o n s , i t was con- s i d e r e d t h a t i f t h e r e p o r t e d mean m o n t h l y r a t e s a n d r e p o r t e d maximum d a i l y r a t e s w e r e t a b u l a t e d , a c h e c k on t h e o r d e r - o f - m a g n i t u d e o f e v a p o i r a t i o n l o s s e s would b e o b t a i n e d . I n a d d i t i o n , t h e r e f e r e n c e s p r o v i d e a u s e f u l b i b l i o g r a p h y on snow e v a p o r a t i o n . T h i s t a b u l a t i o n o f r e f e r e n c e s a n d r e p o r t e d e v a p o r a t i o n r a t e s a r e l i s t e d i n T a b l e 11. A l l t h e r e p o r t e d m e a s u r e m e n t s a p p e a r t o be w i t h i n t h e same g e n e r a l r a n g e . The mean m o n t h l y r a t e s compare f a v o u r a b l y w i t h t h e m o n t h l y r a - t e s c a l c u l a t e d f o r t h e Ottawa s i t e .

D i s c u s s i o n o f R e s u l - t s

I t i s r e a s o n a b l e t o c o n c l u d e t h a t t h e o r d e r - o f - m a g n i t u d e o f snow e v a p o r a t i o n frorn l a r g e snow c o v e r e d a r e a s c a n not7

b e d e t e r m i n e d u s i n g p r e s e n t t e c h n i q u e s , b u t i t i s d i f f i c u l t t o d e t e r m i n e t h e d e g r e e or" a c c u r a c y o f t l i e s e c a l c u l a t e d e v a p o r a t i o n l o s s e s . K a i t e r a f s e x p e r i m e n t s i n d i c a t e t h a t e v a p o r a - t i o n r a t e v a r i e s a g r e a t d e a l w i t h e x p o s u r e ( 1 9 ) . Evapol-ation l o s s e s frorn a s n o w - c o v e r e d s l o p e f a c i n g n o r t h w i l l b e l e s s t h a n f r o m a s i m i l a r s l o p e f a c i n g s o u t h u n d e r cornpal-able c o i i d i t i o n s

.

Eva

pora-Lion l o s s e s from snovt c o v e r i n s h e l t e r e d s i t e s p r o t e c t e d by f o r e s t c o v e r w i l l b e l ~ i u c l ~ l e s s t h a n e v a p o r a t i o n f r o m snow c o v e r i n e x p o s e d s i t e s . f i a p o r a t i o n l o s s e s m u s t b e c a l c u l a - t e d f o r repl-ese11.tative s i t e s w h i c h a r e t y p i c a l o f c o n d i t i o n s o v e r t h e area i n q u e s t i o n .

TABLE I

L i s t of Data u s e d t o C a l c u l a t e Values shown i n F i g u r e 2

Vapour C a l c u l a t e d 3~ Measured

Wind P r e s s u r e D e n s i t y E v a p o r a t i o n E v a p o r a t i o n Date V e l o c i t y D i f f e r e n c e of a i r Rate Rate

Ua e - es gm/cm3 gm/cm /min gm/cm / m i n cm/s e c mb ( X 1 0 - 3 ) (X

1 0 - 4 )

( X

1 0 - 4 )

Dec. 6/56 J a n . 3/57 Jan. 17/57 J a n . 18/57 0 .

osg

0.089 J a n . 28/57 Peb. 11/57 ~ e b .12/57 Feb .19/57 Peb .20/57 Peb .22/57 Average o f 3 _{p a n s}

-

_{450 sq cm i n a r e a}

TABLE I1

Evaporation Rates from Snow during the Spring rVIelt Period

Maximum Daily

Investigator (in. of water/day)

de Quervain (10) Angstrbm (12) Kehrlein (13) Shul%?s (14) ~udovit?s (15) Guy- (16) Poli'%kov

(17

) Chomicz

(18)

Kaitera (19) Pitzgerald (20) Mean Monthly (in. of water/month)

Any formula used to calculate snow evaporation is

limited by the basic accuracy with which the variables of wind speed and vapour pressure can be measured. The measure-

ment of accurate vapour pressure differences by standard

meteorological instruments leaves much to be desired, particularly at low temperatures. The measurement of wind speeds is not too difficult for one site, but it is question- able how representative such observations are of the average conditions required for the calculation of evaporation

losses over a large area. Attempts to estimate evaporation rates from a large area must take into account these factors.

One condition not covered by any evaporation formula is the effect on evaporation when the wind is strong enough to disturb the snow surface. In exposed areas where the wind can move large quantities of snow, this factor might result in evaporation rates considerably higher than a general evaporation equation would indicate. This factor usually is not important when the snow cover is melting as wet snom is cohesive and is not generally moved even by

The o n l y t i m e t h e order-of-magnitude of snow evapora- t i o n amounts might be s i g n i f i c a n t compared t o snow m e l t i s when t h e r e i s a shallokv snow c o v e r and t h e snow-melt p e r i o d

i s extended o v e r s e v e r a l weeks. I n t h i s c a s e , t h e evapora- t i o n from small p o o l s of w a t e r and from exposed s a t u r a t e d s o i l n e a r t h e snow c o v e r might be of importance. W e n t h e snow-melt p e r i o d i s s h o r t , such a s d u r i n g t h e Chinook con- d i t i o n s r e p o r t e d by Hoover ( 2 1 ) , t h e e v a p o r a t i o n from t h e snow s h o u l d be much l e s s t h a n t h e snow m e l t . A s Diamond (1)

p o i n t s o u t , t h e t h e o r y of e v a p o r a t i o n i n d i c a t e s t h a t i n t h e s e c a s e s a much l a r g e r amount of snow

m u s t

be m e l t e d t h a n e v a p o r a t e d .

The order-of-magnitude o f snow e v a p o r a t i o n amounts might be of s i g n i f i c a n c e from a h e a t b a l a n c e p o i n t - o f - v i e w f o r c e r t a i n s p e c i a l i n v e s t i g a t i o n s . Born a h y d r o l o g i c a l v i e w p o i n t , e v a p o r a t i o n l o s s e s from a l a r g e w a t e r s h e d a r e a a r e l i k e l y t o be l e s s t h a n t h e e r r o r s i n o b t a i n i n g samples of t h e t o t a l w a t e r c o n t e n t of t h e snow c o v e r . For t h i s r e a s o n , and because o f t h e g r e a t d i f f i c u l t i e s i n o b t a i n i n g r e p r e s e n t a t i v e measurements of sn.ow e v a p o r a t i o n r a t e s ,

i t

a p p e a r s t h a t h y d r o l o g i s t s

w i l l

have t o be s a t i s f i e d w i t h c a l c u l a t e d e v a p o r a t i o n r a t e s which c a n g i v e a n i n d i c a t i o n of t h e order-of-magnitude of snow e v a p o r a t i o n l o s s e s . Acknowledgments The a u t h o r i s i n d e b t e d t o M r . J. McNally f o r t a k i n g t h e f i e l d measurements a ' t Ottawa, and t o s e v e r a l members of t h e D i v i s i o n o f B u i l d i n g Research f o r h e l p f u l c r i t i c i s m o f t h i s r e p o r t .

R e f e r e n c e s

1. Diamond, b1. E v a p o r a t i o n o r m e l t o f snow c o v e r . U. S. Snow, I c e and. P e r m a f r o s t Resea.rch E s t a b l i s h m e n t , R e s e a r c h P a p e r No. 6 , November 1 3 5 3 .

2. S v e r d r u p , H.U. The e d d y c o n d u c t i v i t y o f t h e a i r o v e r

a smooth snow f i e l d . G e o f y s i s k e P u b l i l c a s j o n e r , Vol. 11, No. 7 , 1 9 3 6 , 6 9 ~ .

3

L i g h t , P. A n a l y s i s o f h i g h r a t e s o f snow m e l t i n g . T r a n s . Amer. Geophys. Union, P t . I , 1 9 4 1 , PP* 195-2050

4.

T h o r n t h w a i t e , C.W. a n d B. Holzman. hTeasurement o f e v a p o r a t i o n f r o m l a n d a n d w a t e r s u r f a c e s . U.S. Dept. A g r i c . , Tech. B u l l e t i n No. 8 1 7 , May 1 9 4 2 , 1 4 3 ~ .

5. A n d e r s o n , E.R. a n d o t h e r s . A r e v i e w o f e v a p o r a t i o n t h e o r y a n d d e v e l o p m e n t o f i n s t r u m e n t a t i o n . U.S. Navy E l e c - t r o n i c Lab.

,

San D i e g o , C a l i f o r n i a

,

A e r o U. S. Navy N.E.L. R e p o r t , 1 3 5 0 .

6 . K o h l e r , M . ,

J.

J.

Nordenson a n d W.E. Pox. E v a p o r a t i o n f r o m p a n s a n d l a k e s . U.S. Dept. o f Commerce, R e s e a r c h P a p e r KO.

38,

May 1 9 5 5 . 7 . Young, A r t h u r A . E v a p o r a t i o n i n v e s t i g a t i o n s i n S o u t h e r n C a l . i f o r n i a . S o i l C o n s e r v a t i o n . S e r v i c e , Uiv. o f I r r i g a t i o n . , Pomona

,

C a l i f . A p r i l 1 3 4 5 . 8 , de Q u e r v a i n , M. E v a p o r a t i o n f r o m t h e snowpack. U. S. Army C o r p s o f E n g i n e e r s , S o u t h P a c i f i c D i v i s i o n , R e s e a r c h K o t e No. 8 , T r a n s l a t i o n . O c t o b e r 1 6 , 1 9 5 2 . 22p. 9 . R e p o r t of t h e Committee o n E v a p o r a t i o n a n d T r a n s p i r a t i o n , 1 9 4 5 - 4 6 . Anlerican G e o p h y s i c a l TJnion. T r a n s .

Amer. Geophys. Union, Vol. 2 7 , O c t o b e r 1 3 4 6 , pp.720-725.

1 0 . C r o f t , A.R. I3vaporat;ion from snow. B u l l e t i n o f t h e American & ~ e t e o r o l o g i c a l S o c i e t y , Vol. 2 5 , 15144, PP* 334-3379

11. Snow h y d r o l o g y . Summary r e p o r t o f t h e snow i n v e s t i g a t i o n s . N o r t h P a c i f i c D i v i s i o n , , U. S. fbmy, Corps o f

Hestr6m,

A n d c r s

=

Measurements o f e v a p o r a - t i o n i n Sweden. Union G e o d e s i q u e ~ e b p h y s , I n t e r n . . A s s o c . H y d r o l o ~ i e S c i . Commission d e P o t a m o l o g i e , 1, Quest. 1, R a p p o r t 6 , W a s h i n g t o n D.C. S e p t e m b e r 1 9 3 9 , 5p. K e h r l e i n , O l i v e r a n d o t h e r s . High S i e r r a snow a b l a t i o n . o b s e r v a t i o n s . P r o c

.

2 1 s t W e s t e r n Snow C o n f e r e n c e 1 9 5 3 , S e p t e m b e r 1 9 5 3 , pp. 47-50. ~ h u l ~ s ~ V J L . S t u d y o x t h e h e a t b a l a n c e o f a snow f i e l d . . (Opyt i z u c h e n i i a t e p l z v o g o b a l a n s a s n e z h n i k a ; T e x t i n R u s s i z n )

.

B i u l l e t e n ' Alcademii Nauk SSSR ( T a s h k e n t ) . No. 11, 1 9 4 6 , pp. 13-16. ~ u d o v i t c \ s , L. Snow e v a p o r a t i o n . (I s p a r e n i e s n e c a ; T e x t i n R u s s i a n )

.

~ e t e o r o l o ~ i c h e s k ~ i Vestnilc, 349 19249 pp.50-52e

Guy, L.T. Snow s t u d i e s o n Bogong High P l a i n s by t h e

S t a t e E l e c t r i c i t y Commission of V i c t o r i a , A u s t r a l i a , I n t e r n a t i o n a l Union Geodesy a n d G,eophysi c s

,

I n t e r n a t i o n a l A s s o c i a t i o n o f S c i e n t i f i c H y d r o l o g y , E d i n b u r g h , S e p t e m b e r 1 9 3 6 , Poli?akov, B . V. T o t a l e v a p o r a t i o n . ( S u g a r n o c i s p a r e n i e ; T e x t i n R u s s i a n ) . G ~ i d ~ o l o g i c h e s k i l A n a l i e i R a s c h e t y , by B.V. P o l i a k o v , L e n i n g r a d , I!dcteoizdat, 1 9 4 6 , pp.126-145.

Chomicz, I(. The i m p o r t a n c e o f snow-cover. s-Ludies f o r w a t e r managemeni;. ( 0 p r z y s t o s o w a n i e bad& n.ad polsrybva s'niekna do p o t r z e b g o s p o d a r k i wad-ne j ; T e x t i n P o l i s h )

.

Gospodarka Wodna

,

1 3 , March 1 9 5 3 , pp.108-111.

K a i t e r a ,

P.

The e v a p o r a t i o n f r o m snow. I n t e r n - a t i o n a l Union o f Geodesy a n d G e o p h y s i c s , I n t e r n a t i o n a l A s s o c i a t i o n o f S c i e n t i f i c H y d r o l o g y , W a s h i n g t o n , D.C. S e p t e m b e r 1 9 3 9 .

F i t z g e r a l d , D. E v a p o r a t i o n . Trans. Am. Soc, C i v i l E n g r s . 1 5 , 1 8 8 6 , pp. 581-646.

H o o v e r , O.H. E f f e c t s of Chinook ( F o e h n ) y i n d s on,snovt c o v e r a n d r u n o f f . I n t

.

Union ~ e ' o d e s i ~ u e G e o p h j ~ s

.

,

I n t e r n . Assoc

.

Hydrologic S c i . Assemblee ~ . e ; ? & r a l e d f O s l o 1 9 - 2 8 a o f i t 1 9 4 8 , v . 2 , pp. 8 6 - 1 0 0

N O T E :

E A C H

P O I N T R E P R E S E N T S

THE AVERAGE R A T E FROM

3 P A N S .

0

8

_{# 5}

_1.8

_1.5

_{2 . 0}

CALCULATED EVAPORATION

grn/crn2/rnin

x

FIGURE

B

RELATIONSHIP

BETWEEN

CALCULATED

EVAPORATION RATE AND MEASURED

EVAPOWA"B0O

R A T E ,

OTTAWA

1956

-57.

C A L C U L A T E D E V A P O R A T I O N R A T E S F R O M